The production of clean distillate products, such as diesel fuels, is becoming more and more important in petroleum processing. This is primarily because governmental regulations are placing ever stricter limits on the amounts of heteroatoms, such as sulfur, as well as other pollutant precursors, which can be present in such products. Conventional hydrotreating processes for removing sulfur from distillates are generally only capable of removing the so-called "easy sulfur" --not the so-called "hard sulfur." "Easy sulfurs" include non-thiophenic sulfur, thiophenes, benzothiophenes, and non-beta-substituted dibenzothiophenes. "Hard sulfurs" include beta-substituted dibenzothiophenes and, in particular, di-beta-substituted dibenzothiophenes. The hard sulfur, which remains after removal of the easy sulfur by conventional hydrodesulfurization, can represent a significant undesirable amount. This amount can be in the range of about 0.2 to 0.3 wt. %, or more of the diesel fraction. In order to meet new governmental regulations, this hard sulfur will also have to be removed from distillate product streams, preferably by the most economic means.
One approach which can be taken to remove hard sulfur is to build new high pressure hydrotreating facilities. While this approach will work from a technology point of view, it is expensive and not cost effective at today's distillate prices. Consequently, there is a critical need in the art for economical methods for removing hard sulfur from distillate product streams in order for these product streams to meet the new regulations.